WO2006/031175 discloses a method for separation of lignin from black liquor comprising the following steps: a) Precipitation of lignin by acidifying black liquor and thereupon dewatering, b) suspending the lignin filter cake obtained in step a) whereupon a second lignin suspension is obtained and adjusting the pH level to approximately the pH level of the washing water of step d) below, c) dewatering of the second lignin suspension, d) addition of washing water and performing a displacement washing at more or less constant conditions without any dramatic gradients in the pH, and e) dewatering of the lignin cake produced in step d) into a high dryness and displacement of the remaining washing liquid in said filter cake, whereby a lignin product is obtained which has an even higher dryness after the displacement washing of step e).
WO2006/038863 discloses a method for precipitating (separation) of lignin, using small amounts of acidifying agents, whereby a lignin product or an intermediate lignin product is obtained which can be used as fuel or chemical feed stock (or as a chemical or a raw material for further refining), from a lignin containing liquid/slurry, such as black liquor. The present invention also provides a method for separation of lignin from a lignin containing liquid/slurry, such as black liquor, whereby a more pure lignin is obtained. Said document also discloses a lignin product or an intermediate lignin product obtainable by the above methods. Said document also discloses use, preferably for the production of heat or for use as chemical, of said lignin product or intermediate lignin product.
When separating lignin from black liquor, which may be achieved through a precipitation in a process, the resulting lignin slurry may be filtered, e.g. in a filter press. The filtrate remaining in the filter cake may cause an increased acid consumption in subsequent process steps. If a large amount of sulphuric acid is added in the process, it can lead to problems with the Na/S balance in the pulp mill as well as increased chemical costs. Therefore, it would be of interest to wash the filter cake and displace the filtrate with another solution. The solubility of lignin is dependent on temperature, ionic strength and pH in the solution (Magnus Norgren, “On the Physical Chemistry of Kraft Lignin, Fundamentals and Applications”, Doctoral Thesis, Physical Chemistry 1, Lund University, 2001). It is not possible to successfully wash lignin by directly applying an acidic wash liquor because of problems with plugging of the filter cake and high yield losses (Fredrik Öhman “Precipitation and separation of lignin from kraft black liquor”, Forest products and Chemical Engineering, Department of Chemical and Biological Engineering, Chalmers University of Technology, Göteborg, Sweden, 2006 and Öhman, F. & Theliander, H., “Washing lignin precipitated from kraft black liquor”, Paperi Ja Puu, Vol. 88, no 5, 287-292 (2006) and Öhman, F., Wallmo, H. & Theliander, H., “A Novel method for washing lignin precipitated from kraft black liquor—Laboratory trails”, Nordic Pulp and Paper Research J., 22 (2007): 1, 9-16
Mixing acidic wash liquor and alkaline filtrate also leads to uncontrolled release of hydrogen sulphide, which can increase the costs of the process.
Accordingly it would be desirable to be able to provide a method for controlling the Na/S balance in a pulp mill when at the same time lignin is separated from black liquor.